In general, since a unit cell of a fuel cell generates too low a voltage to be used alone in practice, the fuel cell has several to several hundred unit cells stacked therein. When stacking the unit cells, a separator or bipolar plate is used to facilitate electrical connection between the unit cells and to separate reaction gases.
The bipolar plate is an essential component of a fuel cell along with a membrane electrode assembly (MEA) and performs a variety of functions such as structural support for the MEA and gas diffusion layer (GDLs), collection and transmission of electric current, transmission and removal of reaction gas, transmission of cooling water used for removal of heat, and the like.
Hence, it is necessary for materials of the separator to have excellent electrical and thermal conductivity, air-tightness, chemical stability, and the like.
Generally, graphite materials and composite graphite materials consisting of a resin and graphite mixture are used to form the separator.
However, graphite materials exhibit lower strength and air-tightness than metallic materials, and suffer from higher manufacturing costs and lower productivity when applied to manufacture of separators. Recently, metallic separators have been actively investigated to overcome such problems of graphite separators.
When a separator is made of a metallic material, there are many advantages in that volume and weight reduction of a fuel cell stack can be accomplished via thickness reduction of the separator, and in that the separator can be fabricated by stamping, which facilitates mass production of the separators.
In this case, however, the metallic material inevitably undergoes corrosion during use of the fuel cell, causing contamination of the MEA and performance deterioration of the fuel cell stack. Further, a thick oxide film can form on the surface of the metallic material after extended use of the fuel cell, causing an increase in internal resistance of the fuel cell.
Stainless steel, titanium alloys, aluminum alloys, nickel alloys, and the like have been proposed as candidate materials for the separator of the fuel cell. Among these materials, stainless steel has received attention for its lower price and good corrosion resistance, but further improvements in corrosion resistance and electrical conductivity are still needed.